css2/visuren.src

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<!-- $Id: visuren.src,v 2.199 2014-01-17 13:47:14 bbos Exp $ -->
<head>
<title>Visual formatting model</title>
<!--script src="http://www.w3c-test.org/css/harness/annotate.js#CSS21_DEV" type="text/javascript" defer></script-->
<style type="text/css">
.current,.proposed,span.delcurrent { background:#feb }
ins.proposed,span.insproposed,li.insproposed { background:#bfb }
del.proposed,span.delproposed { background:#fbb }
span.insproposed,li.insproposed { text-decoration:underline }
span.delproposed,span.delcurrent { text-decoration:line-through }
body>del,body>ins {display:block}
blockquote.math {display: table}
blockquote.math p {display: table-row}
blockquote.math span {display: table-cell; padding-right: 0.3em}
blockquote.math span:first-child {text-align: right}
</style>
</head>
<body>
<h1>Visual formatting model</h1>

<h2><a name="visual-model-intro">Introduction to the visual formatting model</a></h2>

<p>This chapter and the next describe the <span class="index-def"
title="visual formatting model">visual formatting model</span>: how user
agents process the <a href="conform.html#doctree">document tree</a>
for visual <a href="media.html">media</a>.
</p>

<p>In the visual formatting model, each element in the document tree
generates zero or more boxes according to the <a href="box.html">box
model</a>. The layout of these boxes is governed by:</p>

<ul>
<li><a href="box.html#box-dimensions">box dimensions</a> and <a
href="#box-gen">type</a>.
</li><li><a href="#positioning-scheme">positioning scheme</a>
(normal flow, float, and absolute positioning).
</li><li>relationships between elements
in the <a href="conform.html#doctree">document tree.</a>
</li><li>external information (e.g., viewport size, <a
href="conform.html#intrinsic">intrinsic</a> dimensions of images, etc.).
</li></ul>

<p>The properties defined in this chapter and the next apply to both
<a href="media.html#continuous-media-group">continuous media</a> and
<a href="media.html#paged-media-group">paged media</a>. <span class="paged">
However, the
meanings of the <a href="box.html#margin-properties">margin
properties</a> vary when applied to paged media (see the <a
href="page.html#page-margins">page model</a> for details).
</span>
</p>

<p>The visual formatting model does not specify all aspects of
formatting (e.g., it does not specify a letter-spacing algorithm). <a
href="conform.html#conformance">Conforming user agents</a> may behave
differently for those formatting issues not covered by this
specification.
</p>

<h3><a name="viewport">The viewport</a></h3>

<p>User agents for <a
href="media.html#continuous-media-group">continuous media</a>
generally offer users a <span class="index-def" title="viewport">
<dfn>viewport</dfn></span> (a window or other
viewing area on the screen) through which users consult a
document. User agents may change the document's layout when the
viewport is resized (see the <a
href="visudet.html#containing-block-details">initial containing block</a>).
</p>
<p>
When the viewport is smaller than the area of the canvas on which
the document is rendered, the user agent should offer a scrolling
mechanism.

There is at most
one viewport per <a href="intro.html#canvas">canvas</a>, but user
agents may render to more than one canvas (i.e., provide different
views of the same document).
</p>

<h3><span class="index-def" title="containing block">
<a name="containing-block">Containing blocks</a></span>
</h3>

<p>In CSS&nbsp;2.2, many box positions and sizes are calculated with respect
to the edges
of a rectangular box called a <span class="index-def"
title="containing block"><dfn>containing block</dfn></span>.  In
general, generated boxes act as containing blocks for descendant
boxes; we say that a box "establishes" the containing block for its
descendants. The phrase "a box's containing block" means "the
containing block in which the box lives," not the one it generates.
</p>
<p>Each box is given a position with respect to its containing block,
but it is not confined by this containing block; it may <a
href="visufx.html#overflow">overflow</a>.
</p>

<p>The <a href="visudet.html#containing-block-details">details</a> of
how a containing block's dimensions are calculated are described in
the <a href="visudet.html">next chapter</a>.
</p>
<h2><a name="box-gen">Controlling box generation</a></h2>

<p>The following sections describe the types of boxes that may be
generated in CSS&nbsp;2.2. A box's type affects, in part, its behavior in the
visual formatting model. The <span
class="propinst-display">'display'</span> property, described below,
specifies a box's type.
</p>

<p>Certain values of the <span
class="propinst-display">'display'</span> property cause an element of
the source document to generate a <span class="index-def"
title="principal box"><a name="principal-box"><dfn>principal
box</dfn></a></span> that contains descendant boxes and generated
content and is also the box involved in any positioning scheme. Some
elements may generate additional boxes in addition to the principal
box: 'list-item' elements. These additional boxes are placed with
respect to the principal box.

<h3><a name="block-boxes">Block-level elements and block boxes</a></h3>

<p><span class="index-def" title="block-level element"><a
name="block-level"><dfn>Block-level elements</dfn></a></span> &ndash;
those elements of the source document that are formatted visually as
blocks (e.g., paragraphs) &ndash; are elements which generate a
block-level principal box. Values of the <span
class="propinst-display">'display'</span> property that make an
element block-level include: 'block', 'list-item', and 'table'. <span
class="index-def" title="block-level box"><dfn>Block-level
boxes</dfn></span> are boxes that participate in a <a
href="#block-formatting">block formatting context.</a>

<p>In CSS&nbsp;2.2, a block-level box is also a block container box
unless it is a table box or the principal box of a replaced element.
A <span class="index-def" title="block container box"><a
name="block-container-box"><dfn >block container box</dfn></a></span>
either contains only block-level boxes or establishes an <a
href="#inline-formatting" >inline formatting context</a> and thus
contains only <a href="#anonymous-block-level" >inline-level
boxes.</a> An element whose principal box is a block container box is
a <span class="index-def" title="block container element" ><a
name="block-container-element" ><dfn >block container
element.</dfn></a></span> Values of the 'display' property which make
a non-replaced element generate a block container include 'block',
'list-item' and 'inline-block'. Not all block container boxes are
block-level boxes: non-replaced inline blocks and non-replaced table
cells are block containers but are not block-level. Block-level boxes
that are also block containers are called <span class="index-def"
title="block box"><dfn>block boxes</dfn></span>.

<p>The three terms "block-level box," "block container box," and
"block box" are sometimes abbreviated as "block" where unambiguous.

<h4><a name="anonymous-block-level">Anonymous block boxes</a></h4>

<p>In a document like this:
</p>
<pre><code class="html">
&lt;DIV&gt;
  Some text
  &lt;P&gt;More text
&lt;/DIV&gt;
</code></pre>
<p>
(and assuming the DIV and the P both have 'display: block'), the
DIV appears to have both inline content and block content. To make it
easier to define the formatting, we assume that there is an <em><span
class="index-def" title="anonymous">anonymous block box</span></em>
around "Some text".
</p>
<div class="figure">
<p><img src="images/anon-block.png" alt="diagram showing the three
boxes for the example above"> 
</p><p class="caption">Diagram showing the
three boxes, of which one is anonymous, for the example above.
</p></div>

<p>In other words: if a block container box (such as that generated
for the DIV above) has a block-level box inside it (such as the P
above), then we force it to have <em>only</em> block-level boxes
inside it.

<p>When an inline box contains an in-flow block-level box, the inline box
(and its inline ancestors within the same line box) is broken around
the block-level box (and any block-level siblings that are consecutive
or separated only by collapsible whitespace and/or out-of-flow
elements), splitting the inline box into two boxes (even if either
side is empty), one on each side of the block-level box(es). The line
boxes before the break and after the 
break are enclosed in anonymous block boxes, and the block-level box 
becomes a sibling of those anonymous boxes. When such an inline box is 
affected by relative positioning, any resulting translation also
affects the block-level box contained in the inline box.

<div class="example">
<p>This model would apply in the following example if the following
rules:</p>

<pre><code class="css">
p    { display: inline }
span { display: block }
</code></pre>

<p>were used with this HTML document:</p>

<pre class="html-example"><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HEAD&gt;
&lt;TITLE&gt;Anonymous text interrupted by a block&lt;/TITLE&gt;
&lt;/HEAD&gt;
&lt;BODY&gt;
&lt;P&gt;
<em>This is anonymous text before the SPAN.</em>
&lt;SPAN&gt;This is the content of SPAN.&lt;/SPAN&gt;
<em>This is anonymous text after the SPAN.</em>
&lt;/P&gt;
&lt;/BODY&gt;
</code></pre>

<p>The P element contains a chunk (C1) of anonymous text followed
by a block-level element followed by another chunk (C2) of anonymous
text. The resulting boxes would be a block box representing the BODY,
containing an anonymous block box around C1, the SPAN block box, and
another anonymous block box around C2.
</p></div>

<p>The properties of anonymous boxes are inherited from the
enclosing non-anonymous box (e.g., in the example just below the subsection heading "Anonymous block boxes", the one for DIV). 
Non-inherited properties have their initial value. For example,
the font of the anonymous box is inherited from the DIV, but the
margins will be 0.
</p>
<p>
Properties set on elements that cause anonymous block boxes to be 
generated still apply to the boxes and content of that element. For 
example, if a border had been set on the P element in the above 
example, the border would be drawn around C1 (open at the end of the 
line) and C2 (open at the start of the line).
</p>
<p>
Some user agents have implemented borders on inlines containing 
blocks in other ways, e.g., by wrapping such nested blocks inside 
"anonymous line boxes" and thus drawing inline borders around such 
boxes. As CSS1 and CSS2 did not define this behavior, CSS1-only and 
CSS2-only user agents may implement this alternative model and still 
claim conformance to this part of CSS&nbsp;2.2. This does not apply to UAs 
developed after this specification was released.
</p>

<p>Anonymous block boxes are ignored when resolving percentage values
that would refer to it: the closest non-anonymous ancestor box is used
instead. For example, if the child of the anonymous block box inside
the DIV above needs to know the height of its containing block to
resolve a percentage height, then it will use the height of the
containing block formed by the DIV, not of the anonymous block box.

<h3><a name="inline-boxes">Inline-level elements and inline boxes</a></h3>

<p><span class="index-def" title="inline-level element">
<a name="inline-level"><dfn>Inline-level
elements</dfn></a></span> are those elements of the source document that
do not form new blocks of content; the content is distributed in lines
 (e.g., emphasized pieces of text
within a paragraph, inline images,
etc.). The following values of the <span
class="propinst-display">'display'</span> property make an element
inline-level: 'inline', 'inline-table', and 'inline-block'. 

Inline-level elements generate <span class="index-def"
title="inline-level box"><dfn>inline-level
boxes</dfn></span>, which are boxes that participate in an inline
formatting context.

<p>An <span class="index-def" title="inline box"><a
name="inline-box"><dfn>inline
box</dfn></a></span> is one that is both inline-level and whose
contents participate in its containing inline formatting context. A
non-replaced element with a 'display' value of 'inline' generates an
inline box.
Inline-level boxes that are not inline boxes (such as replaced
inline-level elements, inline-block elements, and inline-table
elements) are called <span class="index-def" title="atomic
inline-level box"><dfn>atomic inline-level boxes</dfn></span> because
they participate in their inline formatting context as a single opaque
box.

<h4><a name="anonymous">Anonymous inline boxes</a></h4>

<p>Any text that is directly contained inside a block container element (not
inside an inline element) must be treated as an anonymous inline
element.

<p>In a document with HTML markup like this:</p>

<div class="html-example">
<pre><code class="html">
&lt;p&gt;Some &lt;em&gt;emphasized&lt;/em&gt; text&lt;/p&gt;
</code></pre>
</div>

<p>the <code>&lt;p&gt;</code> generates a block box, with several inline boxes inside
it. The box for "emphasized" is an inline box generated by an inline
element (<code>&lt;em&gt;</code>), but the other boxes ("Some" and "text") are inline boxes generated by a block-level element (<code>&lt;p&gt;</code>). The latter are called <span
class="index-def" title="anonymous inline boxes">anonymous inline
boxes</span>, because they do not have an associated inline-level element.
</p>
<p>Such anonymous inline boxes inherit inheritable properties from
their block parent box. Non-inherited properties have their initial
value. In the example, the color of the anonymous inline boxes is
inherited from the P, but the background is transparent.
</p>

<p>
White space content that would subsequently be collapsed away according to the <span class="propinst-white-space">'white-space'</span> property does not generate any anonymous inline boxes.
</p>

<p>If it is clear from the context which type of anonymous box is
meant, both anonymous inline boxes and anonymous block boxes are
simply called anonymous boxes in this specification.
</p>
<p>There are more types of anonymous boxes that arise when formatting
<a href="tables.html#anonymous-boxes">tables</a>.
</p>
<h3><a name="run-in">Run-in boxes</a></h3>

<p>[This section exists so that the section numbers are the same as in
previous drafts. <span class="index-def" title="run-in">'Display:
run-in'</span> is now defined in CSS level&nbsp;3 (see <a
href="/TR/css3-box">CSS basic box model</a>).]

<h3><a name="display-prop">The</a> <span
class="propinst-display">'display'</span> property</h3>

<!-- #include src=properties/display.srb -->

<p>The values of this property have the following meanings:</p>

<dl>
<dt><span class="index-def" title="'block', definition of"><a
name="value-def-block"><strong>block</strong></a></span>
</dt>
<dd>This value causes an element to generate a principal block box.
</dd>
<dt><span class="index-def" title="'inline-block', definition of"><a
name="value-def-inline-block"><strong>inline-block</strong></a></span>
</dt>
<dd>This value causes an element to generate a principal inline-level block
container.
(The inside of an inline-block is formatted as a block box, and the 
element itself is formatted as an atomic inline-level box.)
</dd>
<dt><span class="index-def" title="'inline', definition of"><a
name="value-def-inline"><strong>inline</strong></a></span>
</dt>
<dd>This value causes an element to generate one or more inline boxes.
</dd>
<dt><span class="index-def" title="'list-item', definition of"><a
name="value-def-list-item"><strong>list-item</strong></a></span>
</dt>
<dd>This value causes an element (e.g., LI in HTML) to generate a
principal block box and a marker box. For information about
lists and examples of list formatting, please consult the section on
<a href="generate.html#lists">lists</a>.
</dd>
<dt><strong>none</strong></dt>

<dd><span class="index-def" title="'none'::as display value">This
value</span> causes an element to not appear in the <a
href="intro.html#formatting-structure">formatting structure</a> (i.e.,
in visual media the element generates no boxes and has no effect on
layout). Descendant elements do not generate any boxes either; the
element and its content are removed from the formatting structure
entirely. This behavior <strong>cannot</strong> be overridden by
setting the <span class="propinst-display">'display'</span> property
on the descendants.

<p>Please note that a display of 'none' does not create an invisible
box; it creates no box at all. CSS includes mechanisms that enable an
element to generate boxes in the formatting structure that affect
formatting but are not visible themselves. Please consult the section
on <a href="visufx.html#visibility">visibility</a> for details.
</p>
</dd>
<dt>
<span class="value-inst-table">table</span>,
<span class="value-inst-inline-table">inline-table</span>,
<span class="value-inst-table-row-group">table-row-group</span>,
<span class="value-inst-table-column">table-column</span>,
<span class="value-inst-table-column-group">table-column-group</span>,
<span class="value-inst-table-header-group">table-header-group</span>,
<span class="value-inst-table-footer-group">table-footer-group</span>,
<span class="value-inst-table-row">table-row</span>,
<span class="value-inst-table-cell">table-cell</span>, and
<span class="value-inst-table-caption">table-caption</span>
</dt>
<dd>These values cause an element to behave like a table element
(subject to restrictions described in the chapter on <a
href="tables.html">tables</a>).
</dd>
</dl>

<p>The computed value is the same as the specified value, except for
positioned and floating elements (see <a
href="#dis-pos-flo">Relationships between 'display', 'position', and
'float'</a>) and for the root element. 
<!-- 
For the root element, the
computed value is as follows: 'inline-table' and 'table' become
'table', 'none' stays 'none', everything else becomes 'block'.
-->
For the root element, the computed value is changed as described in 
the section on the <a href="#dis-pos-flo">relationships between 'display', 'position', and 'float'</a>.
</p>

<p>Note that although the <a href="about.html#initial-value">initial
value</a> of <span class="propinst-display">'display'</span> is
'inline', rules in the user agent's <a
href="cascade.html#default-style-sheet">default style sheet</a> may <a
href="cascade.html">override</a> this value. See the <a
href="sample.html">sample style sheet</a> for HTML 4 in the
appendix.
</p>
<div class="example">
<p>Here are some examples of the <span
class="propinst-display">'display'</span> property:
</p>
<pre><code class="css">
p   { display: block }
em  { display: inline }
li  { display: list-item } 
img { display: none }      /* Do not display images */
</code></pre>
</div>


<h2><a name="positioning-scheme">Positioning schemes</a></h2>

<p>In CSS&nbsp;2.2, a box may be laid out according to three <span
class="index-def" title="positioning scheme"><dfn>positioning
schemes:</dfn></span></p>

<ol>
<li><a href="#normal-flow">Normal flow</a>. In CSS&nbsp;2.2, normal
flow includes <a href="#block-formatting">block formatting</a>
of block-level boxes, 
<a href="#inline-formatting">inline formatting</a>
of inline-level boxes, and <a
href="#relative-positioning">relative positioning</a> of
block-level and inline-level boxes.
</li>
<li><a href="#floats">Floats</a>. In the float model,
a box is first laid out according to the normal flow, then 
taken out of the flow and shifted
to the left or right as far as possible. Content may
flow along the side of a float.
</li>
<li><a href="#absolute-positioning">Absolute positioning</a>. 
In the absolute positioning model, a box is removed from
the normal flow entirely (it has no impact on later siblings)
and assigned a position with respect to a containing block.
</li>
</ol>

<p>An element is called <dfn><span class="index-def">out of
flow</span></dfn> if it is floated, absolutely positioned, or is the
root element. An element is called <dfn><span
class="index-def">in-flow</span></dfn> if it is not out-of-flow. The
<dfn><span class="index-def">flow of an element</span></dfn>
<var>A</var> is the set consisting of <var>A</var> and all in-flow
elements whose nearest out-of-flow ancestor is <var>A</var>.

<div class="note">
<em><strong>Note.</strong>
CSS&nbsp;2.2's positioning schemes help authors make their documents
more accessible by allowing them to avoid mark-up tricks
(e.g., invisible images) used for layout effects.
</em>
</div>

<h3><a name="choose-position">Choosing a positioning scheme:</a> <span
class="propinst-position">'position'</span> property</h3>

<p>The <span class="propinst-position">'position'</span> and <span
class="propinst-float">'float'</span> properties determine which 
of the CSS&nbsp;2.2 positioning algorithms is used to calculate 
the position of a box.
</p>

<!-- #include src=properties/position.srb -->

<p>The values of this property have the following meanings:</p>

<dl>
<dt><strong>static</strong></dt>

<dd>The box is a normal box, laid out according to the <a
href="#normal-flow">normal flow</a>. The 
<span class="propinst-top">'top'</span>, 
<span class="propinst-right">'right'</span>, 
<span class="propinst-bottom">'bottom'</span>,
and <span class="propinst-left">'left'</span>
properties do not apply. 
</dd>
<dt><strong>relative</strong></dt>

<dd>The box's position is calculated according to the <a
href="#normal-flow">normal flow</a> (this is called the position in
normal flow). Then the box is offset <a
href="#relative-positioning">relative</a> to its normal position. When
a box B is relatively positioned, the position of the following box is
calculated as though B were not offset.  The effect of 'position:relative' on table-row-group, table-header-group, table-footer-group, table-row, table-column-group, table-column, table-cell, and table-caption elements is undefined.
</dd>
<dt><strong>absolute</strong></dt>
<dd> The box's position (and possibly size) is specified
with the <span class="propinst-top">'top'</span>,
<span class="propinst-right">'right'</span>, 
<span class="propinst-bottom">'bottom'</span>, and
<span class="propinst-left">'left'</span>
properties.
These properties specify offsets with respect to the box's 
<a href="#containing-block">containing block</a>. Absolutely
positioned boxes are taken out of the normal flow. This means
they have no impact on the layout of later siblings. Also,
though <a href="#absolutely-positioned">absolutely positioned</a> 
boxes have margins, they
do not <a href="box.html#collapsing-margins">collapse</a>
with any other margins.
</dd>
<dt><strong>fixed</strong></dt>
<dd> The box's position is calculated according to the 'absolute'
model, but in addition, the box is <a
href="#fixed-positioning">fixed</a> with respect to some reference.  
As with the 'absolute' model, the box's margins do not collapse with any other margins.
In the case of handheld, projection, screen, tty, and tv media types, 
the box is fixed with respect to the <a href="#viewport">viewport</a> 
and does not move when 
scrolled. In the case of the print media type, the box is rendered on every page, and is fixed with respect to the page box, even if the page is seen through a <a href="#viewport">viewport</a> 
(in the case of a print-preview, for example). For other media 
types, the presentation is undefined. 
Authors may wish to specify 'fixed' in a
media-dependent way.  For instance, an author may want a box to remain
at the top of the <a href="#viewport">viewport</a> on the screen, but
not at the top of each printed page. The two specifications may be
separated by using an <a href="media.html#at-media-rule">@media
rule</a>, as in:

<div class="example">
<pre><code class="css">   
@media screen { 
  h1#first { position: fixed } 
}
@media print { 
  h1#first { position: static }
}
</code></pre>   
</div>
<p>UAs must not paginate the content of fixed boxes. <span
class="note">Note that UAs may print invisible content in other
ways. See <a href="page.html#outside-page-box">"Content outside the
page box"</a> in chapter&nbsp;13.</span>
</dd>
</dl>
<p>
User agents may treat position as 'static' on the root element.
</p>

<h3><a name="position-props">Box offsets</a>: <span
class="propinst-top">'top'</span>, <span
class="propinst-right">'right'</span>, <span
class="propinst-bottom">'bottom'</span>, <span
class="propinst-left">'left'</span></h3>

<p>An element is said to be <span class="index-def" title="positioned
element/box"><a name="positioned-element"><dfn>positioned</dfn></a></span>
if its <span class="propinst-position">'position'</span> property has
a value other than 'static'. Positioned elements generate
positioned boxes, laid out according to four properties:</p>

<!-- #include src=properties/top.srb -->

<p>This property specifies how far an <a
href="#absolutely-positioned">absolutely positioned</a> box's top
margin edge is offset below the top edge of the box's <a
href="#containing-block">containing block</a>. For relatively
positioned boxes, the offset is with respect to the top edges of the
box itself (i.e., the box is given a position in the normal flow, then
offset from that position according to these properties).
</p>

<!-- #include src=properties/right.srb -->

<p>Like 'top', but specifies how far a box's right margin edge is
offset to the left of the right edge of the box's <a
href="#containing-block">containing block</a>.  For relatively
positioned boxes, the offset is with respect to the right edge of the
box itself.
</p>

<!-- #include src=properties/bottom.srb -->

<p>Like 'top', but specifies how far a box's bottom margin edge is
offset above the bottom of the box's <a
href="#containing-block">containing block</a>. For relatively
positioned boxes, the offset is with respect to the bottom edge of the
box itself.
</p>

<!-- #include src=properties/left.srb -->

<p>Like 'top', but specifies how far a box's left margin edge is
offset to the right of the left edge of the box's <a
href="#containing-block">containing block</a>.  For relatively
positioned boxes, the offset is with respect to the left edge of the
box itself.
</p>

<p>The values for the four properties have the following meanings:</p>

<dl>
<dt><strong><span class="value-inst-length">&lt;length&gt;</span></strong></dt>
<dd>The offset is a fixed distance from the reference edge. Negative values are allowed.
</dd>
<dt><strong><span class="value-inst-percentage">&lt;percentage&gt;</span></strong></dt>

<dd>The offset is a percentage of the containing block's width (for <span class="propinst-left">'left'</span> or <span
class="propinst-right">'right'</span>) or height (for <span
class="propinst-top">'top'</span> and <span
class="propinst-bottom">'bottom'</span>). Negative values are allowed.
</dd>
<dt><strong>auto</strong></dt>
<dd>For non-replaced elements, the effect of this value 
depends on which of related properties have the value 'auto' as
well. See the sections on the 
<a href="visudet.html#abs-non-replaced-width">width</a>
and <a href="visudet.html#abs-non-replaced-height">height</a>
of <a href="#absolutely-positioned">absolutely positioned</a>, 
non-replaced elements for details.  For replaced elements, the
effect of this value depends only on the intrinsic dimensions of the
replaced content. See the sections on the <a
href="visudet.html#abs-replaced-width">width</a> and <a
href="visudet.html#abs-replaced-height">height</a> of absolutely
positioned, replaced elements for details.
</dd>
</dl>

<h2><a name="normal-flow">Normal flow</a></h2> 

<p>Boxes in the normal flow belong to a <span class="index-def"
title="formatting context">formatting context</span>, which in CSS&nbsp;2.2
may be table, block or inline. In future levels of CSS, other types of
formatting context will be introduced. <a
href="#block-level">Block-level</a> boxes participate in a <a
href="#block-formatting">block formatting</a> context.  <a
href="#inline-level">Inline-level boxes</a> participate in an <a
href="#inline-formatting">inline formatting</a> context. Table
formatting contexts are described in the <a href="tables.html"
>chapter on tables.</a>
</p>
<h3><a name="block-formatting">Block formatting contexts</a></h3>

<p>Floats, absolutely positioned elements, block containers (such as
inline-blocks, table-cells, and table-captions) that are not block
boxes, and block boxes with 'overflow' other than 'visible'
(except when that value has been propagated to the viewport) establish
new block formatting contexts for their contents.</p>

<p>In a block formatting context, boxes are laid out one after the
other, vertically, beginning at the top of a containing block. The
vertical distance between two sibling boxes is determined by the <span
class="propinst-margin">'margin'</span> properties. Vertical margins
between adjacent block-level boxes in a block formatting context <a
href="box.html#collapsing-margins">collapse</a>.
</p>

<p>
In a block formatting context, each box's left outer edge touches the
left edge of the containing block (for right-to-left formatting, right
edges touch).  This is true even in the presence of floats (although a
box's <em>line boxes</em> may shrink due to the floats), unless the box
establishes a new block formatting context (in which case the box itself
<a href="#bfc-next-to-float"><em>may</em> become narrower</a> due to
the floats).
</p>

<p>For information about page breaks in paged media, please consult
the section on <a href="page.html#allowed-page-breaks">allowed
page breaks</a>.
</p>

<h3><a name="inline-formatting">Inline formatting contexts</a></h3>

<p>An inline formatting context is established by a block container
box that contains no block-level boxes.
In an inline formatting context, boxes are laid out horizontally,
one after the other, beginning at the top of a containing
block. Horizontal margins, borders, and padding are respected between
these boxes. The boxes may be aligned vertically in different ways: their
bottoms or tops may be aligned, or the baselines of text within them
may be aligned. The rectangular area that contains the boxes that form
a line is called a <span class="index-def" title="line box"><a
name="line-box"><dfn>line box</dfn></a></span>. 
</p>
<p>The width of a line box is determined by a <a
href="#containing-block">containing block</a> and the presence of floats.  
The height of a line
box is determined by the rules given in the section on <a
href="visudet.html#line-height">line height calculations</a>. 
</p>
<p>A line box is always tall enough for all of the boxes it contains. 
However, it may be taller than the tallest box it contains 
(if, for example, boxes are aligned so that baselines line up). 
When the height of a box B is less than the height of the line box containing it, 
the vertical alignment of B within the line box is determined by 
the <span class="propinst-vertical-align">'vertical-align'</span> property.
When several inline-level boxes cannot fit horizontally within a single
line box, they are distributed among two or more vertically-stacked
line boxes. Thus, a paragraph is a vertical stack of line boxes. Line
boxes are stacked with no vertical separation (except as specified
elsewhere) and they never overlap.
</p>
<p>In general, the left edge of a line box touches the left edge
of its containing block and the right edge touches the right edge of
its containing block. However, floating boxes may come between the
containing block edge and the line box edge.  Thus, although line
boxes in the same inline formatting context generally have the same
width (that of the containing block), they may vary in width if
available horizontal space is reduced due to <a
href="#floats">floats</a>. Line boxes in the same inline formatting
context generally vary in height (e.g., one line might contain a tall
image while the others contain only text).
</p>
<p>When the total width of the inline-level boxes on a line is less than the
width of the line box containing them, their horizontal distribution
within the line box is determined by the <span
class="propinst-text-align">'text-align'</span> property.  If that
property has the value 'justify', the user agent may stretch spaces
and words in inline boxes (but not inline-table and inline-block
boxes) as well.
</p>

<p>
When an inline box exceeds the width of a line box, it is split into several boxes and these boxes are distributed across several line boxes.  If an inline box cannot be split (e.g., if the inline box contains a single character, or language specific word breaking rules disallow a break within the inline box, or if the inline box is affected by a white-space value of nowrap or pre), then the inline box overflows the line box.
</p>
<p>
When an inline box is split, margins,
borders, and padding have no visual effect where the split occurs (or
at any split, when there are several).
</p>
<p>Inline boxes may also be split into several boxes <em>within the
same line box</em> due to <a href="#direction">bidirectional text
processing</a>.
</p>
<p id="phantom-line-box">Line boxes are created as needed to hold
inline-level content within an inline formatting context. Line boxes
that contain no text, no <a
href="text.html#white-space-prop">preserved white space,</a> no inline
elements with non-zero margins, padding, or borders, and no other <a
href="#positioning-scheme">in-flow</a> content (such as images, inline
blocks or inline tables), and do not end with a preserved newline must
be treated as zero-height line boxes for the purposes of determining
the positions of any elements inside of them, and must be treated as
not existing for any other purpose.
</p>
<div class="html-example"><p> 
Here is an example of inline box construction. The following paragraph
(created by the HTML block-level element P) contains anonymous text
interspersed with the elements EM and STRONG:
</p>
<pre><code class="html">
&lt;P&gt;Several &lt;EM&gt;emphasized words&lt;/EM&gt; appear
&lt;STRONG&gt;in this&lt;/STRONG&gt; sentence, dear.&lt;/P&gt;
</code></pre>

<p>The P element generates a block box that contains five inline
boxes, three of which are anonymous:</p>

<ul>
<li>Anonymous: "Several"</li>
<li>EM: "emphasized words"</li>
<li>Anonymous: "appear"</li>
<li>STRONG: "in this"</li>
<li>Anonymous: "sentence, dear."</li>
</ul>
 
<p>To format the paragraph, the user agent flows the five boxes into
line boxes. In this example, the box generated for the P element
establishes the containing block for the line boxes. If the containing
block is sufficiently wide, all the inline boxes will fit into a
single line box:
</p>
<pre>
 Several <em>emphasized words</em> appear <strong>in this</strong> sentence, dear.
</pre>

<p>If not, the inline boxes will be split up and distributed across
several line boxes. The previous paragraph might be split as follows:
</p>
<pre>
Several <em>emphasized words</em> appear
<strong>in this</strong> sentence, dear.
</pre>

or like this:

<pre>
Several <em>emphasized</em>  
<em>words</em> appear <strong>in this</strong> 
sentence, dear.
</pre>

</div>

<p>In the previous example, the EM box was split into two EM boxes
(call them "split1" and "split2"). Margins, borders,
padding, or text decorations have no visible effect after split1 or
before split2.
</p>
<div class="html-example"><p>
Consider the following example:
</p>
<pre><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Example of inline flow on several lines&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      EM {
        padding: 2px; 
        margin: 1em;
        border-width: medium;
        border-style: dashed;
        line-height: 2.4em;
      }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;Several &lt;EM&gt;emphasized words&lt;/EM&gt; appear here.&lt;/P&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>

<p>Depending on the width of the P, the boxes may be distributed as
follows:</p>

<div class="figure">
<p><img src="./images/inline-layout.png"
     alt="Image illustrating the effect of line breaking on the
display of margins, borders, and padding."></p>
</div>

<ul>
<li> The margin is inserted before "emphasized" and after "words". </li>

<li> The padding is inserted before, above, and below
"emphasized" and after, above, and below "words". A
dashed border is rendered on three sides in each case.</li>
</ul>
</div>

<h3><a name="relative-positioning">Relative positioning</a></h3>  

<p>Once a box has been laid out according to the <a
href="#normal-flow">normal flow</a> or floated, it may be shifted relative to
this position. This is called <span class="index-def" title="relative
positioning"><dfn>relative positioning</dfn></span>. Offsetting a box
(B1) in this way has no effect on the box (B2) that follows: B2 is
given a position as if B1 were not offset and B2 is not re-positioned
after B1's offset is applied. This implies that relative positioning
may cause boxes to overlap.
However, if relative positioning causes an 'overflow:auto' or
'overflow:scroll' box to have
overflow, the UA must allow the user to access this content (at its offset
position), which, through the creation of scrollbars, may affect layout.
</p>
<p>A relatively positioned box keeps its normal flow size, including
line breaks and the space originally reserved for it. The section on
<a href="#containing-block">containing blocks</a> explains when a
relatively positioned box establishes a new containing block.
</p>
<p>For relatively positioned elements, 'left' and 'right' move the
box(es) horizontally, without changing their size. 'Left' moves the
boxes to the right, and 'right' moves them to the left. Since boxes
are not split or stretched as a result of 'left' or 'right', the
used values are always: left = -right.
</p>
<p>If both 'left' and 'right' are 'auto' (their initial values), the
used values are '0' (i.e., the boxes stay in their original
position).
</p>
<p>If 'left' is 'auto', its used value is minus the value of 'right'
(i.e., the boxes move to the left by the value of 'right').
</p>
<p>If 'right' is specified as 'auto', its used value is minus the
value of 'left'.
</p>
<p>If neither 'left' nor 'right' is 'auto', the position is
over-constrained, and one of them has to be ignored. If the 'direction' property of the containing block is 'ltr', the value of 'left' wins and 'right'
becomes -'left'. If 'direction' of the containing block is 'rtl', 'right' wins and 'left' is ignored.
</p>
<div class="example">
<p><strong>Example.</strong> The following three rules are equivalent:
</p>
<pre><code class="css">
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
</code></pre>
</div>

<p>The 'top' and 'bottom' properties move relatively positioned
element(s) up or down without changing their size. 'Top' moves the
boxes down, and 'bottom' moves them up. Since boxes
are not split or stretched as a result of 'top' or 'bottom', the
used values are always: top = -bottom.
If both are 'auto', their used values are both '0'. If one of them is
'auto', it becomes the negative of the other. If neither is 'auto',
'bottom' is ignored (i.e., the used value of 'bottom' will be
minus the value of 'top').
</p>

<p class="note">
Note. 
Dynamic movement of relatively positioned boxes can produce
animation effects in scripting environments (see also the <span
class="propinst-visibility">'visibility'</span> property). 
Although relative positioning may be used as a form of superscripting and 
subscripting, the line height is not automatically adjusted to take the 
positioning into consideration. See the description of <a
href="visudet.html#line-height">line height calculations</a> for more
information.
</p>
<p>Examples of relative positioning are provided in the section <a
href="#comparison">comparing normal flow, floats, and absolute
positioning</a>.
</p>
<h2><a name="floats">Floats</a></h2>

<p>A float is a box that is shifted to the left or right on the
current line. The most interesting characteristic of a float (or
"floated" or "floating" box) is that content may flow along its side
(or be prohibited from doing so by the <span
class="propinst-clear">'clear'</span> property). Content flows down
the right side of a left-floated box and down the left side of a
right-floated box. The following is an introduction to float
positioning and content flow; the exact <a
href="#float-rules">rules</a> governing float behavior are given in
the description of the <span class="propinst-float">'float'</span>
property.
</p>
<p>
A floated box is
shifted to the left or right until its outer edge touches the
containing block edge or the outer edge of another float. 
If there is a line box, the outer top of the floated box is aligned with 
the top of the current line box.
</p>
<p>
If there is not enough horizontal room for the
float, it is shifted downward until either it fits or there are no
more floats present.
</p>
<p>Since a float is not in the flow, non-positioned block boxes
created before and after the float box flow vertically as if the float
did not exist. However,
the current and subsequent
line boxes created next to the float are
shortened
as necessary
to make room for the margin box of the float.

<p>A line box is next to a float when there exists a vertical position
that satisfies all of these four conditions: (a) at or below the top
of the line box, (b) at or above the bottom of the line box, (c) below
the top margin edge of the float, and (d) above the bottom margin edge
of the float.

<p class=note>Note: this means that floats with zero outer height or
negative outer height do not shorten line boxes.

<p>If a shortened line box is too small to contain any content, then
the line box is shifted downward (and its width recomputed) until
either some content fits or there are no more floats present.
Any content in the current line before a floated box is reflowed 
in the same line on the other side of the float.
In other words, if inline-level boxes are placed 
on the line before a left float is encountered that fits in the remaining line box space, 
the left float is placed on that line, aligned with the top of the line box, and then the inline-level boxes already on the line are moved accordingly to the right of the float (the right being the other side of the left float) and vice versa for rtl and right floats.
</p>

<p id="bfc-next-to-float">
The border box of a table, a block-level replaced element, or an element in the normal flow that establishes a new <a href="visuren.html#block-formatting">block formatting context</a> 
(such as an element with 'overflow' other than 'visible')
must not overlap the margin box of any floats in the same block
formatting context as
the element itself. If necessary, implementations should clear the
said element by placing it below any preceding floats, but may place
it adjacent to such floats if there is sufficient space.
They may even make the border box of said element narrower than
defined by <a href="visudet.html#blockwidth">section&nbsp;10.3.3.</a>
CSS2 does not define when a UA may put said element next to the float
or by how much said element may become narrower.
</p>
<div class="example">
<p><strong>Example.</strong> 
In the following document fragment, the containing block is too narrow 
to contain the content next to the float, 
so the content gets moved to below the floats 
where it is aligned in the line box according to the text-align 
property.
</p>
<pre><code class="css">
p { width: 10em; border: solid aqua; }
span { float: left; width: 5em; height: 5em; border: solid blue; }
</code>

...

<code class="html">
&lt;p&gt;
  &lt;span&gt; &lt;/span&gt;
  Supercalifragilisticexpialidocious
&lt;/p&gt;
</code></pre>
<p>This fragment might look like this:</p>
<div class="figure"><p>
<img style="display:block" src="images/supercal.png" 
 alt="Image illustrating the effect of an unbreakable piece of content 
 being reflowed to just after a float which left insufficient room next to it 
 for the content to fit." >
</p>
</div>
</div>

<p>Several floats may be adjacent, and this model also applies to
adjacent floats in the same line.
</p>
<div class="example"><p>
The following rule floats all IMG boxes with
<samp>class="icon"</samp> to the left (and
sets the left margin to '0'):</p>

<pre><code class="css">
img.icon { 
  float: left;
  margin-left: 0;
}
</code></pre>
</div>


<div class="html-example"><p>
Consider the following HTML source and style sheet:</p>

<pre><code class="html">  
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Float example&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      IMG { float: left }
      BODY, P, IMG { margin: 2em }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;&lt;IMG src=img.png alt="This image will illustrate floats"&gt;
       Some sample text that has no other...
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>

<p>The IMG box is floated to the left. The content that follows is
formatted to the right of the float, starting on the same line as the
float. The line boxes to the right of the float are shortened due to
the float's presence, but resume their "normal" width (that of the
containing block established by the P element) after the float.  This
document might be formatted as:</p>

<div class="figure">
<p><img src="./images/floateg.png"
     alt="Image illustrating how floating boxes interact with
margins."></p>
</div>

<p>Formatting would have been exactly the same if the document had
been:
</p>
<pre><code class="html">
&lt;BODY&gt;
  &lt;P&gt;Some sample text 
  &lt;IMG src=img.png alt="This image will illustrate floats"&gt;
           that has no other...
&lt;/BODY&gt;
</code></pre>

<p>because the content to the left of the float is displaced by
the float and reflowed down its right side.
</p></div>

<p>As stated in <a href="box.html#collapsing-margins">section 8.3.1</a>, 
the margins of floating boxes never <a
href="box.html#collapsing-margins">collapse</a> with margins of
adjacent boxes. Thus, in the previous example, vertical margins do not
<a href="box.html#collapsing-margins">collapse</a> between the P box
and the floated IMG box.
</p>
<p>
The contents of floats are stacked as if floats generated new 
stacking contexts, except that any positioned elements and elements
that actually create new
stacking contexts take part in the float's parent stacking context. 
A float can overlap other boxes in the normal flow (e.g., when a 
normal flow box next to a float has negative margins). When this 
happens, floats are rendered in front of non-positioned in-flow 
blocks, but behind in-flow inlines.
</p>
<div class="example">
<p>Here is another illustration, showing what happens when a float
overlaps borders of elements in the normal flow.
</p>
<div class="figure">
<p><img src="images/float2p.png" alt="Image showing a floating image
that overlaps the borders of two paragraphs: the borders are
interrupted by the image.">
</p>
<p class="caption">A floating image obscures borders
of block boxes it overlaps.
</p>
</div>
</div>

<p>The following example illustrates the use of the <span
class="propinst-clear">'clear'</span> property to prevent content
from flowing next to a float.
</p>
<div class="example">
<p>Assuming a rule such as this:
</p>
<pre><code class="css">
p { clear: left }
</code></pre>

<p>formatting might look like this:
</p>
<div class="figure">
<p><img src="images/floatclear.png" alt="Image showing a floating
image and the effect of 'clear: left' on the two paragraphs.">
</p>
<p class="caption">Both paragraphs have set 'clear: left', which
causes the second paragraph to be "pushed down" to a position below
the float &mdash; "clearance" is added above its top margin to
accomplish this (see the <span
class="propinst-clear">'clear'</span> property).
</p>
</div>
</div>


<h3><a name="float-position">Positioning the float:</a> the
<span class="propinst-float">'float'</span> property</h3>

<!-- #include src=properties/float.srb -->

<p>This property specifies whether a box should float to the left,
right, or not at all. It may be set for any element, but only applies to 
elements that generate boxes that are not 
<a href="#absolutely-positioned">absolutely positioned</a>.  
The values of this property have
the following meanings:</p>

<dl>
<dt><strong>left</strong></dt>

<dd>The element generates a <a href="#block-boxes">block</a> box that is
floated to the left. Content flows on the right side of the box,
starting at the top (subject to the <span
class="propinst-clear">'clear'</span> property). 
</dd>
<dt><strong>right</strong></dt>

<dd>Similar to 'left', except the box is floated to the right, and content flows on the left side of the box, starting at the top.</dd>

<dt><strong>none</strong></dt>

<dd>The box is not floated.</dd>
</dl>

<p>
User agents may treat float as 'none' on the root element. 
</p>


<p><span class="index-def" title="float rules"><a
name="float-rules">Here are the precise rules</a></span> that
govern the behavior of floats:</p>

<ol>
<li> The left <a href="box.html#outer-edge">outer edge</a> of a
left-floating box may not be to the left of the left edge of its <a
href="#containing-block">containing block</a>. An
analogous rule holds for right-floating elements.
</li>
<li> If the current box is left-floating, and there are any left-floating 
boxes generated by elements earlier in the source document,
then for each such earlier box, either the left <a
href="box.html#outer-edge">outer edge</a> of the current box must be
to the right of the right <a href="box.html#outer-edge">outer edge</a>
of the earlier box, or its top must be lower than the bottom of the
earlier box. Analogous rules hold for right-floating boxes.
</li>
<li>The right <a href="box.html#outer-edge">outer edge</a> of a
left-floating box may not be to the right of the left <a
href="box.html#outer-edge">outer edge</a> of any right-floating
box that is next to it. Analogous rules hold for
right-floating elements.
</li>
<li>A floating box's <a href="box.html#outer-edge">outer top</a>
may not be higher than the top of its <a
href="#containing-block">containing block</a>.
When the float occurs between two collapsing margins, the
float is positioned as if it had an otherwise empty <a
href="#anonymous-block-level">anonymous block parent</a> taking part in
the flow. The position of such a parent is defined by <a
href="box.html#collapsed-through">the rules</a> in the section on margin
collapsing.
</li>
<li> The <a href="box.html#outer-edge">outer top</a> of a floating box
may not be higher than the outer top of any <a
href="#block-boxes">block</a> or <a
href="#floats">floated</a> box generated by an element
earlier in the source document.
</li>
<li>The <a href="box.html#outer-edge">outer top</a> of an element's
floating box may not be higher than the top of any <span
class="index-inst" title="line-box"><a
href="#line-box">line-box</a></span> containing a box
generated by an element earlier in the source document.
</li>
<li>A left-floating box that has another left-floating box to its left
may not have its right outer edge to the right of its containing
block's right edge. (Loosely: a left float may not stick out at the
right edge, unless it is already as far to the left as possible.) An
analogous rule holds for right-floating elements.
</li>
<li> A floating box must be placed as high as possible.
</li>
<li> A left-floating box must be put as far to the left as
possible, a right-floating box as far to the right as possible. A
higher position is preferred over one that is further to the
left/right.
</li>
</ol>

<p>But in CSS&nbsp;2.2, if, within the block formatting context, there
is an in-flow negative vertical margin such that the float's position
is above the position it would be at were all such negative margins
set to zero, the position of the float is undefined.

<p>References to other elements in these rules refer only to other elements in the same <a href="#block-formatting">block formatting context</a> as the float.
</p>

<div class=example>
<p>This HTML fragment results in the b floating to the right.

<pre>&lt;P>a&lt;SPAN style="float: right">b&lt;/SPAN>&lt;/P></pre>

<p>If the P element's width is enough, the a and the b will be side by
side. It might look like this:

<div class=figure>
<p><img src="images/float-right.png"
alt="An a at the left side of a box and a b at the right side">
</div>
</div>


<h3><a name="flow-control">Controlling flow next to floats:</a>
the <span class="propinst-clear">'clear'</span> property</h3> 

<!-- #include src=properties/clear.srb -->

<p>This property indicates which sides of an element's box(es) may
<em>not</em> be adjacent to an earlier floating box. The 'clear'
property does not consider floats inside the element itself or in
other <a href="visuren.html#block-formatting">block formatting
contexts.</a>
</p>

<p>Values have the following meanings when applied to non-floating
block-level boxes:</p>

<dl>
<dt><strong>left</strong></dt>

<dd>Requires that the top border edge of the box be below the bottom
outer edge of any left-floating boxes that resulted from elements
earlier in the source document.
</dd>

<dt><strong>right</strong></dt>

<dd>Requires that the top border edge of the box be below the bottom
outer edge of any right-floating boxes that resulted from elements
earlier in the source document.
</dd>

<dt><strong>both</strong></dt>

<dd>Requires that the top border edge of the box be below the bottom
outer edge of any right-floating and left-floating boxes that resulted
from elements earlier in the source document.
</dd>

<dt><strong>none</strong></dt>

<dd>No constraint on the box's position with respect to floats.</dd>
</dl>

<p>Values other than 'none' potentially introduce <dfn><span
class="index-def"><a name="clearance">clearance</a></span>.</dfn>
Clearance inhibits margin collapsing
and acts as spacing above the margin-top of an element. It is used to
push the element vertically past the float.

<p>Computing the clearance of an element on which 'clear' is set is
done by first determining the hypothetical position of the element's
top border edge. This position is
where the actual top border edge would have been if the element's
'clear' property had been 'none'.

<p>If this hypothetical position of the element's top border edge is
not past the relevant floats, then clearance is introduced, and
margins collapse according to the rules in 8.3.1.

<p>Then the amount of clearance is set to the greater of:

<ol>
  <li>The amount necessary to place the border edge of the block even
  with the bottom outer edge of the lowest float that is to be
  cleared.
 
  <li>The amount necessary to place the top border edge of the block
  at its hypothetical position.
</ol>

<p>Alternatively, clearance is set exactly to the amount necessary to
place the border edge of the block even with the bottom outer edge of
the lowest float that is to be cleared.

<p class="note"><em><strong>Note:</strong> Both behaviors are allowed
pending evaluation of their compatibility with existing Web content. A
future CSS specification will require either one or the other.</em>

<p class=note>
Note: The clearance can be negative or zero.

<div class=example>
<p>Example 1. Assume (for the sake of simplicity), that we have just
three boxes, in this order: block B1 with a bottom margin of M1 (B1
has no children and no padding or border), floating block F with a
height H, and block B2 with a top margin of M2 (no padding or border,
no children). B2 has 'clear' set to 'both'. We also assume B2 is not
empty.

<p>Without considering the 'clear' property on B2, we have the situation in 
the diagram below. The margins of B1 and B2 collapse. Let's say the 
bottom border edge of B1 is at y = 0, then the top of F is at y = M1, 
the top border edge of B2 is at y = max(M1,M2), and the bottom of F is 
at y = M1 + H.

<div class="figure">
<p><img src="./images/clearance.png"
     alt="Float F extends into the margin above M2."></p>
</div>

<p>We also assume that B2 is not below F, i.e., we are in the
situation described in the spec where we need to add clearance. That
means:

<blockquote>
<p>max(M1,M2) &lt; M1 + H
</blockquote>

<p>We need to compute clearance C twice, C1 and C2, and 
keep the greater of the two: C = max(C1,C2). The first way is to put 
the top of B2 flush with the bottom of F, i.e., at y = M1 + H. That 
means, because the margins no longer collapse with a clearance between 
them:

<blockquote class=math>
<p> <span>bottom of F</span> <span>= top border edge of B2</span> <span>&#8660;</span>
<p>      <span>M1 + H</span> <span>= M1 + C1 + M2</span>          <span>&#8660;</span>
<p>          <span>C1</span> <span>= M1 + H - M1 - M2</span>
<p>            <span></span> <span>= H - M2</span>
</blockquote>

<p>The second computation is to keep the top of B2 where it is, i.e., at y 
= max(M1,M2). That means:

<blockquote class=math>
<p> <span>max(M1,M2)</span> <span>= M1 + C2 + M2</span>          <span>&#8660;</span>
<p>         <span>C2</span> <span>= max(M1,M2) - M1 - M2</span>
</blockquote>

<p>We assumed that max(M1,M2) &lt; M1 + H, which implies

<blockquote class=math>
<p> <span>C2 = max(M1,M2) - M1 - M2</span> <span>&lt; M1 + H - M1 - M2 = H - M2</span> <span>&#8658;</span>
<p>                        <span>C2</span> <span>&lt; H - M2</span>
</blockquote>

<p>And, as C1 = H - M2, it follows that

<blockquote class=math>
<p> <span>C2 &lt; C1</span>
</blockquote>

<p>and hence

<blockquote class=math>
<p> <span>C = max(C1,C2) = C1</span>
</blockquote>
</div>

<div class=example>
<p>Example 2. An example of negative clearance is this situation, in
which the
clearance is -1em. (Assume none of the elements have borders or
padding):

<pre class=html>
&lt;p style="<b>margin-bottom: 4em</b>">
  First paragraph.

&lt;p style="<b>float: left; height: 2em; margin: 0</b>">
  Floating paragraph.

&lt;p style="<b>clear: left; margin-top: 3em</b>">
  Last paragraph.
</pre>

<p>Explanation: Without the 'clear', the first and last paragraphs'
margins would collapse and the last paragraph's top border edge would
be flush with the top of the floating paragraph. But the 'clear'
requires the top border edge to be <em>below</em> the float, i.e., 2em
lower. This means that clearance must be introduced.  Accordingly, the
margins no longer collapse and the amount of clearance is set so that
clearance + margin-top = 2em, i.e., clearance = 2em - margin-top = 2em
- 3em = -1em.
</div>

<p>When the property is set on floating elements, it results in a
modification of the <a href="#float-rules">rules</a> for
positioning the float. An extra constraint (#10) is added:
</p>

<ul>
<li>The top <a href="box.html#outer-edge">outer edge</a> 
of the float must be below the bottom outer
edge of all earlier left-floating boxes (in the case of 'clear:
left'), or all earlier right-floating boxes (in the case of 'clear:
right'), or both ('clear: both').</li>
</ul>

<div class="note"><p>
<em><strong>Note.</strong>
This property <a href="http://www.w3.org/TR/REC-CSS1#clear">applied to all elements in CSS1</a>. Implementations 
may therefore have supported this property on all elements. In CSS2 
and CSS&nbsp;2.2 the 'clear' property only applies to block-level elements. 
Therefore authors should only use this property on block-level
elements.  If an implementation does support clear on inline elements, 
rather than setting a clearance as explained above, 
the implementation should force a break and effectively insert one or more empty line boxes (or shifting the new line box downward as described in <a href="#floats">section 9.5</a>) to move the top of the cleared inline's line box to below the respective floating box(es).
</em>
</p>
</div>

<h2><a name="absolute-positioning">Absolute positioning</a></h2>

<p>In the absolute positioning model, a box is explicitly offset with
respect to its containing block.  It is removed from the normal flow
entirely (it has no impact on later siblings).  An absolutely
positioned box establishes a new containing block for normal flow
children and absolutely (but not fixed) positioned descendants.  However, the contents of an
absolutely positioned element do not flow around any other boxes. They
may obscure the contents of another box (or be obscured themselves), 
depending on the
<a href="#stack-level">stack levels</a> of the overlapping boxes.
</p>
<p>References in this specification to an <span class="index-def"
title="absolutely positioned element">
<a name="absolutely-positioned"><dfn>absolutely positioned
element</dfn></a></span> (or its box) imply that the element's <span
class="propinst-position">'position'</span> property has the value
'absolute' or 'fixed'.
</p>
<h3><a name="fixed-positioning">Fixed positioning</a></h3>

<p>Fixed positioning is a subcategory of absolute positioning. The
only difference is that for a fixed positioned box, the containing
block is established by the <a href="#viewport">viewport</a>.  For <a
href="media.html#continuous-media-group">continuous media</a>, fixed
boxes do not move when the document is scrolled. In this respect, they
are similar to <a href="colors.html#background-properties">fixed
background images</a>.  For <a href="page.html">paged media</a>, boxes
with fixed positions are repeated on every page. This is useful for
placing, for instance, a signature at the bottom of each page.
Boxes with fixed position that are larger than the page area are
clipped. Parts of the fixed position box that are not visible in the
initial containing block will not print.
</p>
<div class="html-example">
<p>Authors may use fixed positioning to create frame-like presentations.
Consider the following frame layout:</p>

<div class="figure">
<p><img src="./images/frame.png"
     alt="Image illustrating a frame-like layout with position='fixed'."></p>
</div>

<p>This might be achieved with the following HTML document and
style rules:</p>

<pre><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;A frame document with CSS&lt;/TITLE&gt;
    &lt;STYLE type="text/css" media="screen"&gt;
      BODY { height: 8.5in } /* Required for percentage heights below */
      #header {
        position: fixed;
        width: 100%;
        height: 15%;
        top: 0;
        right: 0;
        bottom: auto;
        left: 0;
      }
      #sidebar {
        position: fixed;
        width: 10em;
        height: auto;
        top: 15%;
        right: auto;
        bottom: 100px;
        left: 0;
      }
      #main {
        position: fixed;
        width: auto;
        height: auto;
        top: 15%;
        right: 0;
        bottom: 100px;
        left: 10em;
      }
      #footer {
        position: fixed;
        width: 100%;
        height: 100px;
        top: auto;
        right: 0;
        bottom: 0;
        left: 0;
      }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;DIV id="header"&gt; ...  &lt;/DIV&gt;
    &lt;DIV id="sidebar"&gt; ...  &lt;/DIV&gt;
    &lt;DIV id="main"&gt; ...  &lt;/DIV&gt;
    &lt;DIV id="footer"&gt; ...  &lt;/DIV&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>
</div>

<h2><a name="dis-pos-flo">Relationships between 'display', 'position',
and 'float'</a></h2>

<p>The three properties that affect box generation and layout &mdash; 
<span class="propinst-display">'display'</span>, 
<span class="propinst-position">'position'</span>, and 
<span class="propinst-float">'float'</span> &mdash; interact as follows:</p>

<ol>
<li>If <span class="propinst-display">'display'</span>
has the value 'none', then 
<span class="propinst-position">'position'</span> and
<span class="propinst-float">'float'</span> do not apply. 
In this case, the element generates no box.</li>

<li>Otherwise, if <span class="propinst-position">'position'</span>
has the value 'absolute' or 'fixed', the box is absolutely positioned, 
the computed
value of <span class="propinst-float">'float'</span> is 'none',  
and display is set according to the table below.  
The position of the box will be determined by the <span
class="propinst-top">'top'</span>, <span
class="propinst-right">'right'</span>, <span
class="propinst-bottom">'bottom'</span> and <span
class="propinst-left">'left'</span> properties and the box's 
containing block.
</li>

<li>Otherwise, if 'float' has a value other than 'none', the box is
floated and 'display' is set according to the table below.
</li>

<li>Otherwise, if the element is the root element, 
'display' is set according to the table below, except that it is
undefined in CSS&nbsp;2.2 whether a specified value of 'list-item'
becomes a computed value of 'block' or 'list-item'.</li>

<li>Otherwise, the remaining <span
class="propinst-display">'display'</span> property values apply
as specified.
</li>
</ol>

<table border="1">
<tr><th>Specified value </th><th style="width:30%">Computed value</th></tr>
<tr><td>inline-table </td><td>table</td></tr>
<tr><td>inline, table-row-group, table-column,
table-column-group, table-header-group, table-footer-group, table-row,
table-cell, table-caption, inline-block </td><td>block</td></tr>
<tr><td>others </td><td>same as specified</td></tr>
</table>

<h2><a name="comparison">Comparison of normal flow, floats,  
and absolute positioning</a></h2>
<p>To illustrate the differences between normal flow, relative
positioning, floats, and absolute positioning, we provide a series of
examples based on the following HTML:
</p>
<pre class="html-example"><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Comparison of positioning schemes&lt;/TITLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;Beginning of body contents.
      &lt;SPAN id="outer"&gt; Start of outer contents.
      &lt;SPAN id="inner"&gt; Inner contents.&lt;/SPAN&gt;
      End of outer contents.&lt;/SPAN&gt;
      End of body contents.
    &lt;/P&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>

<p>In this document, we assume the following rules:</p>

<pre class="example"><code class="css">
body { display: block; font-size:12px; line-height: 200%; 
       width: 400px; height: 400px }
p    { display: block }
span { display: inline }
</code></pre>

<p>The final positions of boxes generated by the <em>outer</em> and
<em>inner</em> elements vary in each example. In each illustration,
the numbers to the left of the illustration indicate the <a
href="#normal-flow">normal flow</a> position of the double-spaced (for
clarity) lines. </p>

<p class="note">
Note. The diagrams in this section are illustrative and not to 
scale. They are meant to highlight the differences between the 
various positioning schemes in CSS 2.2, and are not intended to be 
reference renderings of the examples given.
</p>

<h3><a name="comp-normal-flow">Normal flow</a></h3>

<p>Consider the following CSS declarations for <em>outer</em> and
<em>inner</em> that do not alter the <a href="#normal-flow">normal
flow</a> of boxes:</p>

<pre class="example"><code class="css">
#outer { color: red }
#inner { color: blue }
</code></pre>

<p>The P element contains all inline content: <a
href="#anonymous">anonymous inline text</a> and two SPAN
elements. Therefore, all of the content will be laid out
in an inline formatting context, within a containing block
established by the P element, producing something like:</p>

<div class="figure">
<p><img src="./images/flow-generic.png"
     alt="Image illustrating the normal flow of text between parent and sibling boxes."></p>
</div>

<h3><a name="comp-relpos">Relative positioning</a></h3>

<p>To see the effect of <a href="#relative-positioning">relative
positioning</a>, we specify:</p>

<pre class="example"><code class="css">
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
</code></pre>

<p>Text flows normally up to the <em>outer</em> element.  The
<em>outer</em> text is then flowed into its normal flow position and
dimensions at the end of line 1. Then, the inline boxes containing the
text (distributed over three lines) are shifted as a unit by '-12px'
(upwards).
</p>

<p>The contents of <em>inner</em>, as a child of <em>outer</em>, would
normally flow immediately after the words "of outer contents" (on line
1.5). However, the <em>inner</em> contents are themselves offset
relative to the <em>outer</em> contents by '12px' (downwards), back to
their original position on line 2.
</p>

<p>Note that the content following <em>outer</em> is not affected by the
relative positioning of <em>outer</em>.</p>

<div class="figure">
<p><img src="./images/flow-relative.png"
     alt="Image illustrating the effects of relative positioning on a
box's content."></p>
</div>

<p>Note also that had the offset of <em>outer</em> been '-24px', the
text of <em>outer</em> and the body text would have overlapped.
</p>

<h3><a name="comp-float">Floating a box</a></h3>

<p>Now consider the effect of <a href="#floats">floating</a> the
<em>inner</em> element's text to the right by means of the following
rules:
</p>
<pre class="example"><code class="css">
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
</code></pre>

<p>Text flows normally up to the <em>inner</em> box, which is pulled
out of the flow and floated to the right margin (its <span
class="propinst-width">'width'</span> has been assigned explicitly).
Line boxes to the left of the float are shortened, and the 
document's remaining text flows into them.</p>

<div class="figure">
<p><img src="./images/flow-float.png"
     alt="Image illustrating the effects of floating a box."></p>
</div>

<p>To show the effect of the <span
class="propinst-clear">'clear'</span> property, we add a <em>sibling</em>
element to the example:
</p>
<pre class="html-example"><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Comparison of positioning schemes II&lt;/TITLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;Beginning of body contents.
      &lt;SPAN id=outer&gt; Start of outer contents.
      &lt;SPAN id=inner&gt; Inner contents.&lt;/SPAN&gt;
      &lt;SPAN id=sibling&gt; Sibling contents.&lt;/SPAN&gt;
      End of outer contents.&lt;/SPAN&gt;
      End of body contents.
    &lt;/P&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>

<p>The following rules:
</p>
<pre class="example"><code class="css">
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
</code></pre>

<p>cause the <em>inner</em> box to float to the right as before and the
document's remaining text to flow into the vacated space:</p>

<div class="figure">
<p><img src="./images/flow-clear.png"
     alt="Image illustrating the effects of floating a box without
setting the clear property to control the flow of text around the
box."></p>
</div>

<p>However, if the <span class="propinst-clear">'clear'</span>
property on the <em>sibling</em> element is set to 'right' (i.e., the
generated <em>sibling</em> box will not accept a position next to
floating boxes to its right), the <em>sibling</em> content begins to
flow below the float:
</p>
<pre class="example"><code class="css">
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
</code></pre>

<div class="figure">
<p><img src="./images/flow-clear2.png" 
alt="Image illustrating the effects of floating an element with setting the clear property to control the flow of text around the element."></p>
</div>

<h3><a name="comp-abspos">Absolute positioning</a></h3>

<p>Finally, we consider the effect of <a
href="#absolute-positioning">absolute positioning</a>.
Consider the following CSS declarations for <em>outer</em> and
<em>inner</em>:</p>

<pre class="example"><code class="css">
#outer { 
    position: absolute; 
    top: 200px; left: 200px; 
    width: 200px; 
    color: red;
}
#inner { color: blue }
</code></pre>

<p>which cause the top of the <em>outer</em> box to be positioned with
respect to its containing block. The containing block for a positioned
box is established by the nearest positioned ancestor (or, if none
exists, the <a href="visudet.html#containing-block-details">initial containing
block</a>, as in our example). The top side of the <em>outer</em> box
is '200px' below the top of the containing block and the left side is
'200px' from the left side. The child box of <em>outer</em> is flowed
normally with respect to its parent.</p>

<div class="figure">
<p><img src="./images/flow-absolute.png"
   alt="Image illustrating the effects of absolutely positioning a box."></p>
</div>

<p>The following example shows an absolutely positioned box that is a
child of a relatively positioned box. Although the parent
<em>outer</em> box is not actually offset, setting its <span
class="propinst-position">'position'</span> property to 'relative'
means that its box may serve as the containing block for positioned
descendants. Since the <em>outer</em> box is an inline box that is
split across several lines, the first inline box's top and left edges
(depicted by thick dashed lines in the illustration below)
serve as references for <span class="propinst-top">'top'</span> and
<span class="propinst-left">'left'</span> offsets.
</p>
<pre class="example"><code class="css">
#outer { 
  position: relative; 
  color: red 
}
#inner { 
  position: absolute; 
  top: 200px; left: -100px; 
  height: 130px; width: 130px; 
  color: blue;
}
</code></pre>

<p>This results in something like the following:</p>

<div class="figure">
<p><img src="./images/flow-abs-rel.png"
     alt="Image illustrating the effects of absolutely positioning a
box with respect to a containing block."></p>
</div>

<p>If we do not position the <em>outer</em> box:</p>

<pre class="example"><code class="css">
#outer { color: red }
#inner {
  position: absolute; 
  top: 200px; left: -100px; 
  height: 130px; width: 130px; 
  color: blue;
}
</code></pre>

<p>the containing block for <em>inner</em> becomes the <a
href="visudet.html#containing-block-details">initial containing block</a> (in our
example). The following illustration shows where the <em>inner</em>
box would end up in this case.</p>


<div class="figure">
<p><img src="./images/flow-static.png"
     alt="Image illustrating the effects of absolutely positioning a box with respect to a containing block established by a normally positioned parent."></p>
</div>

<div class="html-example"><p>
Relative and absolute positioning may be used to implement change
bars, as shown in the following example. The following fragment:
</p>
<pre><code class="html">
&lt;P style=&quot;position: relative; margin-right: 10px; left: 10px;&quot;&gt;
I used two red hyphens to serve as a change bar. They
will &quot;float&quot; to the left of the line containing THIS
&lt;SPAN style=&quot;position: absolute; top: auto; left: -1em; color: red;&quot;&gt;--&lt;/SPAN&gt;
word.&lt;/P&gt;
</code></pre>

<p>might result in something like:</p>

<div class="figure">
<p><img src="./images/changebar.png"
     alt="Image illustrating the use of floats to create a changebar effect."></p>
</div>

<p>First, the paragraph (whose containing block sides are shown in the
illustration) is flowed normally. Then it is offset '10px' from the
left edge of the containing block (thus, a right margin of '10px' has
been reserved in anticipation of the offset).  The two hyphens acting
as change bars are taken out of the flow and positioned at the current
line (due to 'top: auto'), '-1em' from the left edge of its containing
block (established by the P in its final position).  The result is
that the change bars seem to "float" to the left of the current
line.</p>
</div>

<h2><a name="layers">Layered presentation</a></h2>

<h3><a name="z-index">Specifying the stack level</a>: the <span
class="propinst-z-index">'z-index'</span> property</h3> 

<!-- #include src=properties/z-index.srb -->

<p>For a positioned box, the <span
class="propinst-z-index">'z-index'</span> property specifies:

<ol>
  <li>The stack level of the box in the current stacking context.

  <li>Whether the box establishes a stacking context.
</ol>

<p>Values have the following meanings:

<dl>
  <dt><span class="index-inst" title="&lt;integer&gt;"><span
  class="value-inst-integer"><strong>&lt;integer&gt;</strong></span></span>

  <dd>This integer is the stack level of the generated box in the
  current stacking context. The box also establishes a new stacking
  context.

  <dt><strong>auto</strong>

  <dd>The stack level of the generated box in the current stacking
  context is 0. The box does not establish a new stacking context
  unless it is the root element.
</dl>

<p><em>In this section, the expression "in front of" 
means closer to the user as the user faces the screen.</em></p>

<p>In CSS&nbsp;2.2, each box has a position in three dimensions. In addition
to their horizontal and vertical positions, boxes lie along a "z-axis"
and are formatted one on top of the other.  Z-axis positions are
particularly relevant when boxes overlap visually. This section
discusses how boxes may be positioned along the z-axis.
</p>
<p>
The order in which the rendering tree is painted onto the canvas is 
described in terms of stacking contexts. Stacking contexts can 
contain further stacking contexts. A stacking context is atomic from 
the point of view of its parent stacking context; boxes in other 
stacking contexts may not come between any of its boxes.
</p>
<p>Each box belongs to one <span class="index-def" title="stacking
context"><dfn>stacking context</dfn></span>. Each positioned box in a
given stacking context has an integer <span class="index-def"
title="stack level"><a name="stack-level"><dfn>stack
level</dfn></a></span>, which is its position on the z-axis relative
other stack levels within the same stacking context. Boxes with
greater stack levels are always formatted
in front of boxes with lower stack levels.  Boxes may have negative
stack levels.  Boxes with the same stack level in a stacking context
are stacked back-to-front according to document tree order.
</p>

<p>
The root element forms the root stacking context. Other stacking 
contexts are generated by any positioned element (including 
relatively positioned elements) having a computed value of 'z-index' 
other than 'auto'. Stacking contexts are not necessarily related to 
containing blocks. In future levels of CSS, other properties may 
introduce stacking contexts, for example '<a
href="http://www.w3.org/TR/css3-color/#transparency">opacity</a>'
[[-CSS3COLOR]].
</p>

<p>Within each stacking context, the following layers are painted in
back-to-front order:

<ol>
  <li>the background and borders of the element forming the stacking
  context.

  <li>the child stacking contexts with negative stack levels (most
  negative first).

  <li>the in-flow, non-inline-level, non-positioned descendants.

  <li>the non-positioned floats.

  <li>the in-flow, inline-level, non-positioned descendants, including
  inline tables and inline blocks.

  <li>the child stacking contexts with stack level 0 and the
  positioned descendants with stack level 0.

  <li>the child stacking contexts with positive stack levels (least
  positive first).
</ol>

<p>Within each stacking context, positioned elements with stack level
0 (in layer&nbsp;6), non-positioned floats (layer&nbsp;4), inline
blocks (layer&nbsp;5), and inline tables (layer&nbsp;5), are painted
as if those elements themselves generated new stacking contexts,
except that their positioned descendants and any would-be child
stacking contexts take part in the current stacking context.

<p>This painting order is applied recursively to each stacking
context. This description of stacking context painting order
constitutes an overview of the detailed normative definition in
<a href="zindex.html">Appendix&nbsp;E.</a>

<div class="html-example">
<p>In the following example, the stack levels of
the boxes (named with their "id" attributes) are:
"text2"=0, "image"=1, "text3"=2, and "text1"=3. The
"text2" stack level is inherited from the root box. The
others are specified with the <span
class="propinst-z-index">'z-index'</span> property.
</p>
<pre><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Z-order positioning&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      .pile { 
        position: absolute; 
        left: 2in; 
        top: 2in; 
        width: 3in; 
        height: 3in; 
      }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;
      &lt;IMG id="image" class="pile" 
           src="butterfly.png" alt="A butterfly image"
           style="z-index: 1"&gt;

    &lt;DIV id="text1" class="pile" 
         style="z-index: 3"&gt;
      This text will overlay the butterfly image.
    &lt;/DIV&gt;

    &lt;DIV id="text2"&gt;
      This text will be beneath everything.
    &lt;/DIV&gt;

    &lt;DIV id="text3" class="pile" 
         style="z-index: 2"&gt;
      This text will underlay text1, but overlay the butterfly image
    &lt;/DIV&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>
</div>


<p>This example demonstrates the notion of
<em>transparency</em>. The default behavior of the background is to allow boxes behind it to be visible. 
In the example, each box transparently overlays the boxes below it. This
behavior can be overridden by using one of the existing
<a href="colors.html#background-properties">
background properties</a>.
</p>

<h2><a name="direction">Text direction:</a>
the <span class="propinst-direction">'direction'</span>
and <span class="propinst-unicode-bidi">'unicode-bidi'</span>
properties
</h2>

<p><a href="conform.html#conformance">Conforming</a> user agents that do
not support bidirectional text may ignore the <span
class="propinst-direction">'direction'</span> and <span
class="propinst-unicode-bidi">'unicode-bidi'</span> properties
described in this section. This exception includes UAs that render
right-to-left characters simply because a font on the system contains them
but do not support the concept of right-to-left text direction.

<p>The characters in certain scripts are written from right to
left. In some documents, in particular those written with the Arabic
or Hebrew script, and in some mixed-language contexts, text in a
single (visually displayed) block may appear with mixed
directionality. This phenomenon is called <span class="index-def"
title="bidirectionality (bidi)"><dfn>bidirectionality</dfn></span>, or
"bidi" for short.
</p>
<p>The Unicode standard ([[UNICODE]], [[UAX9]]) defines a complex
algorithm for determining the proper directionality of text. The
algorithm consists of an implicit part based on character properties,
as well as explicit controls for embeddings and overrides. CSS&nbsp;2.2 relies
on this algorithm to achieve proper bidirectional rendering. The <span
class="propinst-direction">'direction'</span> and <span
class="propinst-unicode-bidi">'unicode-bidi'</span> properties allow
authors to specify how the elements and attributes of a document
language map to this algorithm.
</p>
<p>User agents that support bidirectional text must apply the Unicode
bidirectional algorithm to every sequence of inline-level boxes uninterrupted
by a forced (<a
href="http://www.unicode.org/reports/tr9/#Bidirectional_Character_Types">bidi
class&nbsp;B</a>) break or block boundary. This sequence forms the
"paragraph" unit in the bidirectional algorithm. The paragraph embedding
level is set according to the value of the <span
class="propinst-direction">'direction'</span> property of the containing
block rather than by the heuristic given in steps P2 and P3 of the Unicode
algorithm.</p>
<p>Because the directionality of a text depends on the structure and
semantics of the document language, these properties should in most
cases be used only by designers of document type descriptions (DTDs),
or authors of special documents. If a default style sheet specifies
these properties, authors and users should not specify rules to
override them. 
</p>
<p>The HTML 4 specification ([HTML4], section 8.2) defines
bidirectionality behavior for HTML elements. The style sheet
rules that would achieve the bidi behavior specified in [[HTML4]] are
given in <a href="sample.html#bidi">the sample style sheet</a>. The
HTML 4 specification also contains more information on
bidirectionality issues.
</p>

<!-- #include src=properties/direction.srb -->

<p>This property specifies the base writing direction of blocks and
the direction of embeddings and overrides (see <span
class="propinst-unicode-bidi">'unicode-bidi'</span>) for the Unicode
bidirectional algorithm. In addition, it specifies such things as the
direction of <a
href="tables.html">table</a> column layout, the direction of
horizontal <a href="visufx.html#overflow">overflow</a>, the
position of an incomplete last line in a block in case of 'text-align:
justify'.
</p>

<p>Values for this property have the following meanings:</p>

<dl>
<dt><strong>ltr</strong></dt>
<dd>Left-to-right direction.</dd>
<dt><strong>rtl</strong></dt>
<dd> Right-to-left direction.</dd>
</dl>

<p>For the <span class="propinst-direction">'direction'</span>
property to affect reordering in inline elements, the <span
class="propinst-unicode-bidi">'unicode-bidi'</span> property's value
must be 'embed' or 'override'.
</p>
<div class="note"><p>
<em><strong>Note.</strong> 
The <span class="propinst-direction">'direction'</span> property, when
specified for table column elements, is not inherited by cells in the
column since columns are not the ancestors of the cells in the document tree. 
Thus, CSS cannot easily capture the "dir" attribute inheritance rules described
in [[HTML4]], section 11.3.2.1.
</em></p></div>

<!-- #include src=properties/unicode-bidi.srb -->

<p>Values for this property have the following meanings:</p>

<dl>
<dt><strong>normal</strong></dt>
<dd>The element does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline elements,
implicit reordering works across element boundaries.
</dd>
<dt><strong>embed</strong></dt>

<dd>If the element is inline, this value
opens an additional level of embedding with respect to the
bidirectional algorithm. The direction of this embedding level is
given by the <span class="propinst-direction">'direction'</span>
property. Inside the element, reordering is done implicitly. This
corresponds to adding a LRE (U+202A; for 'direction: ltr') or RLE
(U+202B; for 'direction: rtl') at the start of the element and a PDF
(U+202C) at the end of the element.
</dd>
<dt><strong>bidi-override</strong></dt>

<dd>
For inline elements this creates an override.
For block container elements 
this creates an override for inline-level descendants not within 
another block container
element.
This means that inside the element, reordering is strictly in sequence
according to the <span class="propinst-direction">'direction'</span>
property; the implicit part of the bidirectional algorithm is
ignored. This corresponds to adding a LRO (U+202D; for 'direction:
ltr') or RLO (U+202E; for 'direction: rtl') at the start of the
element or at the start of each anonymous child block box, if any, and
a PDF (U+202C) at the end of the element.
</dd>
</dl>

<p>The final order of characters in each block container is the
same as if the bidi control codes had been added as described above,
markup had been stripped, and the resulting character sequence had
been passed to an implementation of the Unicode bidirectional
algorithm for plain text that produced the same line-breaks as the
styled text.
In this process, replaced elements with 'display: inline'
are treated as
neutral characters, unless their <span
class="propinst-unicode-bidi">'unicode-bidi'</span> property has a
value other than 'normal', in which case they are treated as strong
characters in the <span class="propinst-direction">'direction'</span>
specified for the element.
All other atomic inline-level boxes are treated as neutral characters
always.


</p>
<p>Please note that in order to be able to flow inline boxes in a
uniform direction (either entirely left-to-right or entirely
right-to-left), more inline boxes (including anonymous inline boxes)
may have to be created, and some inline boxes may have to be split up
and reordered before flowing.
</p>
<p>Because the Unicode algorithm has a limit of 
<em title="According to unicode 3.0, chapter 3, section 12, definition BD2. 
Specifically, page 58 here: http://www.unicode.org/unicode/uni2book/ch03.pdf" 
>61 levels</em> of
embedding, care should be taken not to use <span
class="propinst-unicode-bidi">'unicode-bidi'</span> with a value other
than 'normal' unless appropriate. In particular, a value of 'inherit'
should be used with extreme caution. However, for elements that are,
in general, intended to be displayed as blocks, a setting of
'unicode-bidi: embed' is preferred to keep the element together in
case display is changed to inline (see example below).
</p>
<p>The following example shows an XML document with bidirectional
text. It illustrates an important design principle: <span
class="index-inst" title="DTD">DTD</span> designers should take bidi
into account both in the language proper (elements and attributes) and
in any accompanying style sheets. The style sheets should be designed
so that bidi rules are separate from other style rules. The bidi rules
should not be overridden by other style sheets so that the document
language's or DTD's bidi behavior is preserved.
</p>
<div class="example"><p>
In this example, 
lowercase letters stand for inherently left-to-right characters and
uppercase letters represent inherently right-to-left characters:
</p>
<pre class="xml-example"><code class="xml">
&lt;HEBREW&gt;
  &lt;PAR&gt;HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5&lt;/PAR&gt;
  &lt;PAR&gt;HEBREW6 &lt;EMPH&gt;HEBREW7&lt;/EMPH&gt; HEBREW8&lt;/PAR&gt;
&lt;/HEBREW&gt;
&lt;ENGLISH&gt;
  &lt;PAR&gt;english9 english10 english11 HEBREW12 HEBREW13&lt;/PAR&gt;
  &lt;PAR&gt;english14 english15 english16&lt;/PAR&gt;
  &lt;PAR&gt;english17 &lt;HE-QUO&gt;HEBREW18 english19 HEBREW20&lt;/HE-QUO&gt;&lt;/PAR&gt;
&lt;/ENGLISH&gt;
</code></pre>

<p>Since this is XML, the style sheet is responsible for setting the
writing direction. This is the style sheet:</p>

<pre class="example">
/* Rules for bidi */
HEBREW, HE-QUO  {direction: rtl; unicode-bidi: embed}
ENGLISH         {direction: ltr; unicode-bidi: embed} 

/* Rules for presentation */
HEBREW, ENGLISH, PAR  {display: block}
EMPH                  {font-weight: bold}
</pre>

<p>The HEBREW element is a block with a right-to-left base direction,
the ENGLISH element is a block with a left-to-right base
direction. The PARs are blocks that inherit the base direction from
their parents. Thus, the first two PARs are read starting at the top
right, the final three are read starting at the top left.  Please note
that HEBREW and ENGLISH are chosen as element names for explicitness
only; in general, element names should convey structure without
reference to language.
</p>
<p>The EMPH element is inline-level, and since its value for <span
class="propinst-unicode-bidi">'unicode-bidi'</span> is 'normal' (the
initial value), it has no effect on the ordering of the text. The
HE-QUO element, on the other hand, creates an embedding.
</p>
<p>The formatting of this text might look like this if the line length
is long:
</p>
<pre class="ascii-art">
               5WERBEH 4WERBEH english3 2WERBEH 1WERBEH

                                8WERBEH <b>7WERBEH</b> 6WERBEH

english9 english10 english11 13WERBEH 12WERBEH

english14 english15 english16

english17 20WERBEH english19 18WERBEH
</pre>

<p>Note that the HE-QUO embedding causes HEBREW18 to be to the right
of english19.
</p>
<p>If lines have to be broken, it might be more like this:
</p>
<pre class="ascii-art">
       2WERBEH 1WERBEH
  -EH 4WERBEH english3
                 5WERB

   -EH <b>7WERBEH</b> 6WERBEH
                 8WERB

english9 english10 en-
glish11 12WERBEH
13WERBEH

english14 english15
english16

english17 18WERBEH
20WERBEH english19
</pre>

<p>Because HEBREW18 must be read before english19, it is on the line
above english19. Just breaking the long line from the earlier
formatting would not have worked. Note also that the first syllable
from english19 might have fit on the previous line, but hyphenation of
left-to-right words in a right-to-left context, and vice versa, is
usually suppressed to avoid having to display a hyphen in the middle
of a line.
</p>
</div><!-- example -->

</body>
</html>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:nil
sgml-shorttag:nil
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:nil
sgml-indent-data:t
sgml-parent-document:nil
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
-->